The invention relates to photodiodes. In particular the invention relates to photodiodes having high bandwidth-efficiency products used in optical network receivers.
Semiconductor photodetectors, most notably various forms of photodiodes, absorb incident light in the form of photons and convert the absorbed photons into an electric current. The current within a lattice of the semiconductor is often represented in terms of xe2x80x98free carriersxe2x80x99 or simply xe2x80x98carriersxe2x80x99. In particular, when a photon with sufficient energy interacts with an atom of the semiconductor lattice, an electron associated with the atom moves across an energy band gap from a valence shell or band to a conduction shell or band of the semiconductor. Movement of the electron across the band gap creates a negative carrier, i.e., the electron, and leaves behind a positive carrier known as a xe2x80x98holexe2x80x99. After carrier generation through photon absorption, a carrier transport mechanism within the semiconductor-based photodetector separates the generated holes and electrons, thereby creating an electric current known generally as a photocurrent. In general, both the electron and the hole may act as carriers within the semiconductor and contribute to the photoelectric current. The photocurrent thus created enables"" the photodetector to interact in various ways with an external circuit or system. Among other things, photodiodes find wide-scale application in optical receivers used for optical communication networks.
Photodetector performance is often summarized in terms of bandwidth, efficiency, maximum current output, and optical wavelength range. Bandwidth is a measure of a speed of response of the photodetector to changes in an incident optical signal or light source. Efficiency indicates how much of the incident optical signal is converted into carriers. Maximum current output is typically determined by a saturation condition within the semiconductor of the photodetector while optical wavelength range is a function of certain material properties of the photodetector among other things. In general, photodetector performance is limited by a combination of material properties of constituent materials of the photodetector and a structural characteristic of the photodetector associated primarily with a type and/or structure of a given photodetector.
For example, FIG. 1A illustrates a cross sectional view of a conventional positive-intrinsic-negative (PIN) photodiode 10. The PIN photodiode 10 comprises an intrinsic or lightly doped semiconductor layer 14 (i-layer) sandwiched between a p-type semiconductor layer 12 (p-layer) and an n-type semiconductor layer 15 (n-layer). The i-layer 14 is often referred to as a photoactive or a light absorption layer 14 since ideally, photon absorption is primarily confined to the i-layer 14 of the PIN diode 10. Typically a deposited metal, such as aluminum (Al), or another conductive material, such as heavily doped polysilicon, form a pair of ohmic contacts 17a, 17b, that provide an electrical connection between the PIN photodiode and an external circuit.
The ohmic contact 17a connected to the p-layer 12 is an anode contact 17a while the ohmic contact 17b connected to the n-layer 15 is a cathode contact 17b. Typically, the PIN photodiode 10 is formed on and structurally supported by a semi-insulating substrate 19.
FIG. 1B illustrates a band diagram 20 of the PIN photodiode 10 illustrated in FIG. 1A. The band diagram 20 depicts energy levels as electron-volts (eV) in a vertical or y-direction and physical length or distance along a conduction path within a device in a horizontal or x-direction. Thus, the band diagram 20 illustrates a valence band energy level 21 and a conduction band energy level 22 separated by a band gap 23 for each of the layers of the PIN photodiode 10. When a hole 30 and electron 32 are separated by absorption of a photon by the photoactive i-layer 14, the hole 30 moves in the i-layer 14 toward the p-layer 12 under the influence of an electric potential gradient formed by an inherently lower energy level of the p-layer 12 for holes. Once the hole reaches the p-layer 12, the hole combines at the anode ohmic contact 17a with an electron supplied by the external circuit (not illustrated). Similarly, the electron 32 moves in the i-layer 14 toward the n-layer 15 under the influence of an electric potential gradient formed by the inherently lower energy level of the n-layer 15 for electrons. Electrons in the n-layer 15 enter the cathode contact 17b. The drift or movement of electrons 32 and holes 30 in the i-layer 14 drives or creates an electric current in the n-layer 15, the p-layer 12, and the external circuit.
Among the performance limitations associated with the conventional PIN photodiode is a bandwidth limitation due to the time required for the transport of holes 30 and electrons 32 within the i-layer 14. In particular, holes 30 are known to have a much slower transport velocity than that of electrons 32. The slower transport velocity of holes 30 results in a transport time for the holes 30 that is much longer than a transport time of the electrons 32. The longer hole transport time normally dominates and ultimately limits an overall response time or bandwidth of the PIN photodiode 10.
Accordingly, it would be advantageous to have a photodiode that overcomes the bandwidth-efficiency product limitations associated with conventional PIN photodiodes. Such a photodiode would solve a longstanding need in the area of photodiodes for optical networking.
The present invention provides an extended drift heterostructure (EDH) photodiode with enhanced electron response. In particular, the present invention is an enhanced EDH (EEDH) photodiode that employs an additional p-type light absorption layer. The additional p-type light absorption layer promotes unidirectional photo-generated minority carrier (e.g., electron) drift or motion within the photodiode according to the present invention. The unidirectional electron carrier motion effectively enhances an electron contribution to a device photocurrent without degrading an overall device bandwidth.
In an aspect of the invention, an enhanced extended drift heterostructure (EEDH) photodiode is provided. The EEDH photodiode comprises a first layer comprising a semiconductor having a first doping concentration that maintains a charge neutrality condition in at least a portion of the first layer. The EEDH photodiode further comprises a second layer adjacent and interfaced to the first layer. The second layer comprises a semiconductor having a second doping concentration that is lower than the first doping concentration, such that a non-neutral charge condition is maintained. The first and second layers comprise respective first and second band gap energies that facilitate light absorption by the first and second layers. The EEDH photodiode further comprises an ohmic anode contact directly or indirectly interfaced to the first layer and a cathode contact directly or indirectly interfaced to the second layer. A characteristic of one or more of the layers in addition to the second layer directs a movement of photo-generated electrons away from the ohmic anode contact.
In some embodiments, the characteristic that directs the movement of the photo-generated electrons is manifested in a carrier block layer adjacent and interfaced to the first layer on a side opposite to the second layer. The carrier block layer comprises a semiconductor having a block band gap energy that is greater than the first and second band gap energies, such that a block energy barrier is created between the first layer and the carrier block layer to so direct the electron movement. In other embodiments, the characteristic that so directs the electron movement is either further manifested in the first layer or alternatively manifested in the first layer. The first layer has the first band gap energy and the first doping concentration, either or both of which is graded to produce a quasi-field. The quasi-field preferentially moves the photo-generated electrons toward the second layer.
Further in some embodiments of the present invention, the cathode contact comprises a Schottky cathode contact interfaced to the second layer. In other embodiments, the cathode contact comprises an ohmic cathode contact interfaced to the second layer and a contact layer between the ohmic cathode contact and the second layer. The contact layer comprises a semiconductor in a second conduction type relative to a first conduction type of the semiconductor of the first layer.
In yet another aspect of the present invention, a method of constructing the EEDH photodiode of the present invention is provided.